1. Field of the Invention
The present invention relates to a method of setting the origin of a linear motor. More particularly, the invention relates to a method of setting an origin in a linear motor that incorporates a magnetic linear encoder used as high-speed repeated positioning means.
2. Description of the Related Art
Linear motors are used as drive sources in many transfer mechanisms and transporting mechanisms. When a linear motor is used, it is necessary to detect where the movable member is moving in the linear motor. A linear encoder is used as means for detecting the position of the movable member. A method is employed, in which the linear motion of the movable member is converted to a rotation, and a rotary encoder detects the position of the movable member from the rotation. Either type of a encoder, i.e., linear encoder or rotary encoder, first detects magnetic marks or optical marks arranged at regular intervals, then multiplies the marks detected, and performs interpolation division on any two adjacent marks, thereby detecting how long the movable member has moved.
These encoders can indeed detect the distance the movable member has moved, or the incremental value of the movable member. However, they cannot detect the absolute position of the member (i.e., the position the member assumes with respect to a coordinate origin). In order to detect the absolute position, a position must be set, which can be used as a reference for the multiplication of marks or interpolation signals. That is, the position of an origin for the movable member needs to be established.
Hitherto, a method has been employed to establish the position of such an origin in a linear motor. In the method, a limit switch is provided at a desirable position on the side of the fixed unit of the linear motor, the movable member of the motor is moved in a prescribed direction, and the position where the limit switch detects the dog mounted on the movable member is set as origin.
A method of setting an origin in a linear motor, which uses no means such as a limit switch or a dog, has been proposed (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 8-322276). In this method, the operator first positions a linear encoder, used as position detector, parallel to the linear motor and then moves the movable member of the motor, placing the member at a predetermined position, and moves the movable member in a prescribed direction from the position at low speed and then resets a reversible counter for calculating the present position of the movable member at a position where the polarity of a magnetic signal from a hall element is changed by a set number. The position the linear encoder (i.e., position detector) has detected by resetting the counter is used as origin in the linear motor.
In linear motors using an optical linear encoder, the detection sensibility of the encoder decreases if the dust or dirt sticks to it, disabling the linear motor from maintaining a sufficient resistance to environmental changes. In view of this, a linear motor has been proposed, which is resistant to environmental changes, which can be manufactured at low cost and in which the magnetic linear encoder used need not be set (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-56892). In this linear motor, the permanent magnets work not only as the field system of the motor, but also as magnetic scale unit that is to be detected by the linear encoder, and are arranged at such a pitch as would serve as the scale pitch of the magnetic scale unit. This linear motor is therefore resistant to dust and environmental changes.
In the method of setting or establishing the origin by using a limit switch and a dog, the operator needs to adjust the positions of the limit switch and the position of the dog.
In the method of establishing an origin, disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 8-322276, the operator utilizes a mark in order to set the origin at a position. At first, the operator roughly sets the origin at such a position that the mark put to the movable member may roughly align within the width of a permanent magnet with the two marks put to the fixed unit. Thereafter, an origin will be set or established in the linear motor. Thus, the operator must work to set the origin at all. In other words, men need to perform various setting operations. In practice, the origin cannot be automatically set in any linear motors.
In the linear motor disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2004-56892, no setting is required in the magnetic linear encoder. The scale head of the magnetic linear encoder comprises a plurality of Hall elements so arranged to have a phase difference of 90° in terms of electrical angle with respect to each other. The Hall elements output two-phase, sine-wave analog signals, which are supplied to a position-data converter. The position-data converter converts these signals to position data, from which the position the movable member takes at present is calculated. Jpn. Pat. Appln. Laid-Open Publication No. 2004-56892 shows, in FIG. 2A, the waveform of an induced voltage and the waveform of an encoder signal. The induced voltage and the encoder signal can have such waveforms only if they have been generated from those of the magnets, which constitute the middle part of the column composed of permanent magnets arranged at regular intervals. In other words, they cannot have such waveforms if generated from the permanent magnets that constitute either end part of the magnet column. Hence, an origin that serves to establish the absolute position of the movable member in the linear motor cannot be set or established, merely by processing the waveforms of the encoder signal. Obviously, the linear motor disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2004-56892 is based on the premises that a means or a method must be used to set the origin for a position-data converter that establishes the absolute position of the movable member. In Jpn. Pat. Appln. Laid-Open Publication No. 2004-56892, nothing is described of the motor origin or the origin setting.